A metal powder sintered compact has been adopted as one of filters employed in the chemical industry, the polymer industry, the chemicals industry and others. As metals herein, there have been used generally brass, stainless steel and, recently, titanium.
Titanium is greatly excellent in corrosion resistance, acid proofness and the like as compared with stainless steel, but on the other hand, extremely poor in moldability. Hence, generally, a titanium sintered filter has been fabricated according to a method in which hydrogenation/dehydrogenation titanium powder accepted as having a comparatively good moldability is molded with a die press, followed by sintering, and furthermore, a method disclosed in JP A 1995(H7)-238302 uses titanium sponge powder comparatively good in moldability similarly to hydrogenation/dehydrogenation powder.
Such titanium sintered filters have started finding applications thereof to, for example, highly corrosion resistant filters for a carrier gas inlet section of a gas chromatography apparatus, production of food such as a liquid condiment, and a liquid pigment.
Filters well used in various application fields have been faced a demand for maximum pore diameters adapted for respective purposes of usage. The term, “a maximum pore diameter” is used as an index expressing the size of a particle removable by a filter, wherein with the same value of a maximum pore diameter, it may be considered that filters having respective pore shapes different from each other can remove particles having at least the same diameter. A filter with the smaller pressure drop is more requested among filters with the same maximum pore diameter. For example, as the carrier gas inlet section filter for a gas chromatography apparatus, a filter has been desired that is not only excellent in corrosion resistance but also especially has a maximum pore diameter of 70 μm or less and a smaller pressure drop.
In a titanium sintered filter using hydrogenation/dehydrogenation powder or titanium sponge powder, however, there exists a constraint to disable a pressure drop to be reduced to a small value in a case where a maximum pore diameter is adjusted to 70 μm or less.
A titanium sintered filter using hydrogenation/dehydrogenation powder or titanium sponge powder has another problem that the filter is very hard and fragile without flexibility; therefore it is easy to be broken if being thin and difficult to fabricate the filter large in area. In addition, since bending is difficult at room, temperature, a product cannot be fabricated by bending, causing a problem of a high fabrication cost except for a plate-like shape.
For example, a case arises where there is requested a titanium sintered filter of the shape of a cylinder, of the order of 40 mm in diameter (with a radius of curvature of 20 mm), whereas since it is impossible to bend a titanium sintered compact in the form of a flat plate into the form of a cylinder at room temperature, a necessity arises for working by cold isostatic pressing called CIP for short as described in JP. No. 2791737, thereby increasing in fabrication cost cannot be avoided.
Even with hydrogenation/dehydrogenation titanium powder and titanium sponge powder, moldability is inferior to that of stainless steel. Hence, it is difficult to mold the titanium powders into shapes except for a thin flat plate. Therefore, it is also difficult to directly mold a filter in the shape of a cylinder without resorting to bending process.
That is, in a case where a sintered compact in the shape of a cylinder is fabricated by press molding using hydrogenation/dehydrogenation titanium powder or titanium sponge powder, a press force in a direction of height does not effectively act, which results in difficulty in molding a middle portion in the direction of height; therefore a cylinder with a large height cannot be produced though a low-profile ring can be produced. While a cylinder with a large height can be fabricated by cold isostatic pressing called CIP instead of a press, a high cost is encountered, making CIP improper as a fabrication method for a sintered filter. Therefore, while it is imagined to stack rings along the central axis direction to weld the rings, needless to say that thus fabricated sintered filter is much higher in cost as compared with a sintered filter fabricated by press molding stainless steel powder.
Incidentally, a method is described in JP No. 2791737 in which stainless steel powder is subjected to cold isostatic pressing too fabricate a sintered filter in the shape of a cylinder.
As a different problem of a titanium sintered filter using hydrogenation/dehydrogenation titanium powder or titanium sponge powder, low reverse-washing property arises. That is, sizes and shapes of cavities are randomly distributed in titanium sintered filters made of each powder. While a filter of this kind is used over a long term repeating reverse-washing, if sizes and shapes of cavities are random, solid matter trapped therein are not sufficiently removed even with reverse-washing. Hence, the problem of low reverse-washing reproducibility has remained.
It is described in JP No. 2791737 that, as for a stainless steel filter, a diameter of a cavity is increased in a direction from the front surface to the rear surface of a stainless steel sintered filter in order to enhance reverse-washing reproducibility of a metal powder sintered filter. To be concrete, a slurry obtained by dispersing fine powder into a binder resin solution is applied on the surface of a porous compact obtained by presintering and thereafter, sintering of the porous compact is conducted, thereby forming a skin layer with fine pores on the surface of the porous compact.
In such a multilayer structure, since almost all solid matter in a treated liquid is trapped in the skin layer with fine pores formed therein and no foreign matter is trapped in cavities inside the base layer, the solid matter trapped and accumulated in the skin layer is easy to be removed by reverse-washing. On the other hand, the following problem arises however.
Stainless steel is inferior to titanium in corrosion resistance. Furthermore, stainless steel powder used here is composed of irregularly shaped particles produced by a water atomization technique; therefore, sizes and shapes of cavities in a sintered compact are random in not only the base layer but also the skin layer. Besides, since the skin layer does not receive the action of press molding though the base layer receives the action of press molding, sizes and shapes of fine pores in the skin layer are more random than in the base layer. For this reason, solid matter remains in the skin layer after reverse-washing, thereby disabling reverse-washing reproducibility as high as expectation. In addition, since a void ratio of the skin layer receiving no press molding is largely different from that of the base layer receiving press molding, there also arises a risk that permeability of a treated liquid is lowered.
A sintering compact is also used as a power feeder in a water electrolytic cell producing hydrogen and oxygen using a polymer electrolytic film. Concrete description will be given of the water electrolytic cell; a construction is generally employed in which a unit is formed by placing power feeders on both surfaces of a film electrode laminates formed by laminating catalyst layers onto both surfaces of the polymer electrolyte film, multiple of units are stacked and electrodes are provided on both sides thereof.
The power feeders herein are made each of a porous conductive plate and placed in close contact with an adjacent film electrode laminates. Why a porous conductive plate is used as a power feeder is that a current is required to flow through, that water is required to be supplied for a water electrolytic reaction and that gas generated in the water electrolytic reaction is quickly expelled out.
A structure of a fuel cell using a polymer electrolytic film is also the same as that of the water electrolyzer and porous conductive plates are placed on both surfaces of a film electrode laminates. In the case of a fuel cell, since an electric power is obtained with hydrogen as fuel, the porous conductive plates are called current collectors.
As to a porous conductive plate such as a power feeder in such a polymer electrolyte membrane type water electrolyzer or a current collector in such a solid polymer fuel cell, titanium have been studied because of a necessity for characteristics enabling a material to be used in an oxidizing atmosphere, and among titanium with actual natures and conditions, especially a sintered compact has drawn attention since a surface is smooth, it is difficult to damage an adjacent film electrode laminates and a proper void ratio can be attained with ease.
As porous conductive plates made of a titanium sintered compact, there are exemplified: a titanium powder sintered plate obtained by sintering powder obtained by crushing titanium sponge or powder produced by pulverizing titanium sponge by hydrogenation and dehydrogenation thereof; a titanium fiber sintered plate obtained by compression molding titanium fibers to sinter the preform; and a titanium fiber sintered plate on a surface of which a plasma sprayed layer of metallic titanium is formed, the last of which is disclosed in JP A 1999(H11)-302891.
A porous conductive plate made of a prior art titanium sintered compact described above, however, has the following problems.
Though a titanium powder sintered compact has an advantage that it is smooth on surfaces thereof and gives no damage on an adjacent film electrode laminates, the sintered compact has a fatal restraint that it is poor in press moldability and easily broken; therefore, it cannot be fabricated with a small thickness and a large area. On the other hand, though a titanium fiber sintered plate is good in moldability and can be produced with a small thickness and a large area, it has acute angled protrusions and depressions on surfaces thereof with large spacings between fibers. Therefore, if titanium fiber sintered plates are brought into press contact with an adjacent film electrode laminates, there is a high risk to damage the film electrode laminates. Furthermore, there remains a problem to increase a contact resistance between the titanium fiber sintered plate and the film electrode laminates.
In contrast to the sintered compacts described above, a titanium fiber sintered plate disclosed in JP A 1999(H11)-302891 is a sintered plate in which a plasma sprayed layer of metallic titanium is formed on a surface of the titanium fiber sintered plate to thereby cancel acute angled protrusions and depressions, and large spacings between fibers, and can be said to be excellent in moldability thereof and contactability between the sintered plate and the film electrode laminates.
Since in addition to requirement of an extra cost due to plasma spraying, avoid ratio and surface profiles of a titanium fiber sintered plate are different from those of a plasma sprayed titanium layer on the surface of the plate, an electric resistance increases at a bonding interface therebetween, leading to an electric resistance of a porous conductive plate higher than to be expected from an apparent void ratio. As a result, in a water electrolytic cell used at a high current density, for example, in the range of from 1 to 3 A/cm2, a large voltage drop results. Needless to say that such a voltage drop is not at all allowed in a fuel cell.
Moreover, a large change in void ratio at a bonding interface leads to a worry to adversely influence on permeability of a gas and a liquid.
On the other hand, as an ink dispersion plate for a large ink jet printer, there has been requested a porous plate of, for example, a thickness of as thin as 2 mm or less and an area of as large as 200 mm×100 mm or more. This porous plate requires a small variation in void ratio from a nature of this kind. As such an ink dispersion plate, there has been used a sintered plate made of irregular shaped powder of stainless steel.
As the recent trend, a demand has started to be generated on a porous plate more excellent in corrosion resistance than a sintered plate of stainless steel powder, for which it is considered to use titanium powder more excellent in corrosion resistance than stainless steel.
Though titanium in greatly excellent in corrosion resistance and acid proofness as compared with stainless steel, it is extremely poor in moldability to the contrary. Hence, a general fabrication method for a titanium sintered plate has been thought to be such that hydrogenation/dehydrogenation titanium powder, which has been accepted to be comparatively good in moldability, is molded with a die press, followed by sintering the preform and furthermore, another fabrication method is also described in JP A 1995(H7)-238302 in which there is used titanium sponge powder, which is comparatively good in moldability, similar to the case of hydrogenation/dehydrogenation titanium powder.
Moreover, a different method is described in JP A 1996(H8)-170107 in which a metal powder sintered plate uniform in void ratio is fabricated by HIP.
The present inventors tried a procedure in which hydrogenation/dehydrogenation titanium powder or titanium sponge powder is molded with a die press to sinter the preform for the purpose to fabricate a dispersion plate uniform in void ratio with a thickness as thin as 2 mm or less and an area as large as 200 mm×100 mm or more, and since the dispersion plate was excessively thin, it was broken after press, having disabled fabrication thereof.
The present inventors tried fabrication of the dispersion plate described above by HIP only to find difficulty. The reason for the difficulty is that a porous plate after sintering was not able to be separated from a capsule maintaining a shape of a sintered compact during HIP. Moreover, it is also difficult to select a material of which the capsule is made, which together with the above reason, causes a fabrication cost to be raised to a very high value.
The present invention has been made in light of such circumstances and it is a first object to provide a titanium powder sintered compact excellent in corrosion resistance, having a small maximum pore diameter, and showing a performance of a small pressure drop during usage as a sintered titanium filter.
It is a second object of the present invention to provide a titanium powder sintered compact excellent in bending.
It is a third subject of the present invention to provide a cylindrical porous compact low in fabrication cost despite using titanium powder, and excellent in reverse-washing reproducibility while being used as a powder sintered filter.
It is a fourth object of the present invention to provide a metal sintered filter excellent in corrosion resistance and reverse-washing reproducibility.
It is a fifth object of the present invention to provide a porous conductive plate excellent not only naturally in moldability, but also in surface smoothness even without coating like plasma spraying, and in addition, easy in production and excellent in economy.
It is a sixth object of the present invention to provide a highly corrosion resistant porous plate capable of economically satisfying a condition to realize a uniform void ratio and a small thickness as required by an ink dispersion plate for use in a large ink jet printer.